Cased heat resistant alloy to reduce mercury corrosion

ABSTRACT

Mercury corrosion of the interior of closed metal vessels, such as bulbs of temperature responsive devices, which contain mercury at elevated temperatures and pressures is reduced by constructing the walls of such vessels of a heat resistant ferrous alloy which is essentially free of elements, such as chromium and nickel, that are relatively soluble in hot mercury and by casing the outer periphery of the walls normally exposed to an oxidizing atmosphere with an element such as chromium and/or nickel.

United States Patent 1 Shopsky CASED HEAT RESISTANT ALLOY TO REDUCE MERCURY CORROSION [75] Inventor: Harvey J. Shopsky, Latrobe, Pa.

[73] Assignee: Robertshaw Controls Company,

Richmond, Va.

[22] Filed: Apr. 26, 1972 [21] Appl. No.: 247,773

[52] 11.8. C1. 73/368, 29/195 M, 117/107, 431/66 [51] Int. Cl. G01k 1/08, GOlk 5/00 [58] Field of Search 73/371, 368; 148/635; 117/107; 23/252; 431/66; 29/195 M; 220/64 [56] References Cited UNITED STATES PATENTS 2,175,771 10/1939 Giles 117/107 X 2,366,091 12/1944 Eskin 73I368.4 2,415,309 2/1947 Stone.. 23/252 X 2,442,223 5/1948 Uhlig., 148/635 2,725,748 12/1955 Liberatore 73/371 1 May 28, 1974 2,739,907 3/1956 Nowak 148/635 2,858,244 10/1958 Long et al 148/635 X 3,127,283 3/1964 Chadwick 117/107 X 3,213,922 10/1965 Weber 73/368.4 X 3,218,861 ll/1965 Moore et a1. 73/371 Primary Examiner-Richard C. Queisser Assistant Examiner-Daniel M. Yasich Attorney, Agent, or Firm-Anthony A. OBrien [5 7] ABSTRACT 3 Claims, No Drawings BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the reduction of mercury corrosion in closed metal vessels, such as bulbs of temperature responsive devices, which contain mercury at high temperatures and high internal pressures.

Description of the Prior Art Mercury is utilized in a number of high temperaturehigh pressure applications, such as in power generation apparatus and in temperature control devices. Mercury has a number of desirable physical properties including a low melting point, high volumetric expansion, vapor pressure, flow ability and stability at high temperatures. In addition, mercury is readily available, low cost and relatively easy to handle.

Unfortunately, however, mercury is difficult to contain in metal vessels at high temperatures and pressures due to its ability to dissolve most common metals. It has a particular affinity for nickel and chromium which are elements required for oxidation resistance in steel alloys at service temperatures between 1000 and 2000F. The oxidation resistance and service temperature at which these alloys may be used is generally proportional to the amount of nickel or chromium in the alloy. On the other hand, it has been found that the solubility of these two elements, i.e. nickel and chromium, is an exponential function of the temperature of the mercury. In addition, it has been found that the rate of corrosion or solubility of chromium and nickel in mercuryis proportional to the percentage of these elements in the steel alloy.

The utilization of mercury in various high temperature-high pressure applications is shown in prior art patents. For example, U.S. Pat. No. 2,366,091 is concerned with the use of special steel alloys to avoid mercury corrosion. U.S. Pat. No. 2,640,313 deals with a mercury filled sensing device.

SUMMARY OF THE INVENTION The present invention is embodied in an apparatus including a closed metal vessel, such as a bulb of a temperature responsive device, containing mercury in which the walls of the vessel are constructed basically of a ferrous alloy which is essentially free of elements, such as nickel and chromium, which are relatively soluble in hot mercury and the outer surface of the walls is cased with nickel, chromium or a combination thereof, whereby mercury corrosion is minimized at high temperatures and high internal pressures.

A primary object of the present invention is to reduce mercury corrosion of closed metal vessels at high temperatures and pressures.

A further object of the present invention is to increase the service and material life of closed metal vessels containing mercury at high temperatures and pressures.

A still further object of the present invention is to increase the permissible operating temperature of closed metal vessels containing mercury at high temperatures and pressures.

Other objects and advantages of the present invention will become apparent from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is advantageously used in temperature responsive devices which conventionally include a temperature sensing bulb containing mercury which is connected in fluid communication with a capillary tube which in turn is connected in fluid communication to an actuating element. These devices are well known, for example, in the art of temperature responsive controlswhich are suitable for high temperature use such as in the detection of flame at a burner.

The temperature sensing bulb of these temperature responsive devices is constructed of a ferrous or steel alloy which is heat resistant and which as initially constructed is essentially free of elements, such as chromium and nickel, which are relatively soluble in hot mercury and wherein the outer surface is cased with chromium, nickel or other elements or combinations thereof which increase the oxidation resistance of the bulb when exposed to a high temperature flame in an oxidizing atmosphere. Thus, the cased bulb should be capable of operating satisfactorily at elevated temperatures in the range of 1000 to 2000F and at high internal pressures of the magnitude experienced when mercury is heated to these temperatures in a closed vessel. Oxidation on the interior of the bulb is not a problem since normally the air or oxygen is evacuated before mercury is added to the bulb.

The casing of the exterior surface of the bulb may be accomplished by an alphatizing process which will cause the chromium, for example, to permeate the outer surface to a controlled depth. The finished exterior case will provide basically the same oxidizing resistance as a steel alloy with the same percentage of chromium throughout its entire structure. The outer surface of the bulb may be cased by conventional techniques such as disclosed, for example, in U.S. Pat. Nos. 2,874,070 and 3,127,283.

In the process described in U.S. Pat. No. 2,874,070, a superficial alloy of chromium may be formed on an iron body by heating a cementation mixture capable of forming chromium fluoride vapors on heating, heating the iron body to reach and maintain a temperature at which chromium from the vapors will diffuse into the surface of the iron body and bringing the chromium vapors in contact with the iron body surface while maintaining the relative temperatures of the vapors and the iron body surface to prevent condensation of chromium fluoride on the iron body.

In the process described in U.S. Pat. No. 3,127,283 a metal containing substrate may be cased with chromium, nickel or certain other elements. In this process the substrate and the casing material are introduced into a casing zone and heated at a reduced pressure to a point where they have a substantial mutual affinity, the temperature being below the temperature at which the substrate melts and below the temperature at which the vapor pressure of the absorbed casing material at the surface of the substrate is equal to the vapor pressure of the casing material, and intimately contacting the substrate with the casing material vapor for a time until the vapor has diffused into the substrate to a predetermined depth.

It has been found that the present invention reduces mercury corrosion to a minimum in an application such as temperature control of a gas or oil burner. This can be accomplished while using common steel materials for construction of the devices of the present invention. Also, the service and material life of the devices is substantially increased. The present invention also permits the temperature responsive devices to be utilized at a higher temperature than conventional devices without sacrificing service life due to increased mercury corrosion.

The present invention will eliminate sigma phase embrittlement which is common to straight-chromium steels containing sufficient chromium to prevent oxidation at 1600F. Sigma phase is a complex intermetallic chromium-iron compound that is hard, nonmagnetic and extremely brittle at room temperature. It forms when ferritic and austenitic stainless steels containing more than about 16.5 percent chromium are heated for a long time between 1 100 and 1700F. Also, the present invention will eliminate 885F embrittlement which is common to ferritic chromium stainless steels containing more than about 13 percent chromium. The 885F. embrittlernent is a room temperature brittleness that occurs in the prementioned alloy after exposures to temperatures between 750 and 1050F. for an extended period of time.

Generally speaking, it is preferred to case the outer surface of the bulb under treating conditions which will produce a surface containing in the range of about 50 to 70 percent chrome with iron as the balance. A chrome content of about 60 percent is particularly preferred. It is to be noted that the chrome cased surface is an alloy, rather than simply a coating of chromium. Case depth varies with time and temperature of treatment and chemistry of the metal being treated as is well known in the art. Good scaling resistance may be obtained with a case depth as little as 0.001 inch for temperatures less than l400F. Case depths of 0.006 inches or more may be utilized.

in accordance with the present invention a temperature responsive device was constructed wherein the bulb was made of 502 stainless steel alloy having a case depth inthe range of about 0.001 to 0.006 inch and containing about 60 percent chrome on the chromized surface. This device of the present invention was then tested in comparision with a conventional temperature responsive device wherein the bulb was made of stainless steel AISl type 446. Mercury corrosion was determined by measuring the thinnest wall section by X-ray. The tests have shown to date that whereas the conventional device failed under test conditions after about 12 months, the device of the present invention has not failed after more than 18 months, although the rate of corrosion increased after 12 months.

Inasmuch as the present invention is subject to many 5 modifications, variations and changes in detail, it is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A temperature responsive device capable of withstanding an elevated temperature in a range of about l000 to 2000F comprising an evacuated temperature sensing bulb for use in a temperature responsive means,

said bulb containing mercury as a fluid responsive to temperature changes,

said bulb being made from a ferrous alloy essentially free of nickel, chromium and other elements relatively soluble in mercury when subjected to the elevated temperature, and

said bulb having an outer surface into which is diffused a casing of chromium, nickel or a combination of nickel and chromium sufficient to render the outer surface of the bulb oxidation resistant at the elevated temperature, the inner surface of said bulb being essentially free of chromium and nickel and being resistant to mercury corrosion at the elevated temperature.

2. A temperature responsive device capable of withstanding an elevated temperature in the range of about 1000 to 2000F with a minimum of mercury corrosion which comprises an evacuated temperature sensing bulb for use in a 3 temperature responsive means,

said bulb containing mercury as a fluid responsive to temperature changes,

said bulb being made from a ferrous alloy essentially free of nickel, chromium and other elements relatively soluble in mercury when subjected to the elevated temperature,

said bulb having an outer surface which is cased by diffusing sufficient chromium to a controlled depth therein to render the outer surface of the bulb oxidation resistant at the elevated temperature, the controlled depth being such as to leave the inner surface of the bulb essentially free of chromium.

3. A temperature responsive device as defined in claim 2 wherein the chromium in said outer surface of the bulb has a concentration within the range from 50 to 70 percent. 

2. A temperature responsive device capable of withstanding an elevated temperature in the range of about 1000* to 2000*F witH a minimum of mercury corrosion which comprises an evacuated temperature sensing bulb for use in a temperature responsive means, said bulb containing mercury as a fluid responsive to temperature changes, said bulb being made from a ferrous alloy essentially free of nickel, chromium and other elements relatively soluble in mercury when subjected to the elevated temperature, said bulb having an outer surface which is cased by diffusing sufficient chromium to a controlled depth therein to render the outer surface of the bulb oxidation resistant at the elevated temperature, the controlled depth being such as to leave the inner surface of the bulb essentially free of chromium.
 3. A temperature responsive device as defined in claim 2 wherein the chromium in said outer surface of the bulb has a concentration within the range from 50 to 70 percent. 